1. Field of the Invention
The present invention relates to an automatic brake control apparatus which is built-in a braking system for an airplane, vehicles or the like, and a brake pressure control valve suitably used therefor.
2. Description of the Related Art
A conventional automatic brake has been used mainly as a brake for the airplane. In FIG. 4, an automatic brake control apparatus for an airplane is shown. The control apparatus is provided with an electro-hydraulic servovalve 51, a pump 52 for feeding oil pressure thereto, and a controller 53 for outputting a control signal to the electro-hydraulic servovalve 51. The controller 53 is given a speed reduction rate selecting signal from a setting device 54 and the number of rotations of the wheels from a rotation number sensor 56 provided at a wheel 55. The number of rotations of the wheels is used for computing the speed reduction rate, but recently a G-sensor 58 may directly be used for the control input.
When the condition of road surface (a dry, wet or icy road surface) is selected by a switch or the like, a desired speed reduction rate corresponding to the condition is selected by the controller 53. When the automatic brake operates, a control signal is sent from the controller 53 to the electro-hydraulic servovalve 51 so that an actual speed reduction rate coincides with the desired rate, and the brake pressure is applied from the electro-hydraulic servovalve 51 to the brake 57 on the basis of the control signal.
The electro-hydraulic servovalve which is conventionally used for the automatic brake control apparatus is advantageous in that a response frequency is high, 100 HZ or more, and adjustment pressure almost proportional to an input current is obtainable, but has certain defects. For example, the conventional servovalve is high in manufacturing cost, does not withstand contamination caused by operating oil, is sensitive to magnetic fields because it is operated by a weak magnetic force of a torque motor unit, always requires operating oil, has high oil consumption and large hysteresis between the input current and the adjusting pressure.
The conventional automatic brake control apparatus, which directly compensates for various defects of the electrohydraulic servovalve is expensive to produce and low in durability. Also, the apparatus, when used as an emergency brake, continuously circulates the operating oil by use of a hydraulic pump. Therefore, the use of the automatic brake control apparatus has been limited to expensive systems such as large-scale passenger plane brake systems.